Interactive effects of soil fertility and initial community states on small-scale plant species richness

Background/Question/Methods

Plant ecologists have long sought to understand the multiple processes governing plant species coexistence and patterns of variation in species richness within and among communities. Much work has focused on exploring patterns of richness along environmental gradients of soil fertility, disturbance and plant productivity. However, a variety of stochastic and historical factors that produce variation in local species pools for colonization and initial community states can potentially generate alternative assembly trajectories independent of environmental gradients. Such historical phenomena may not only limit the explanatory power of the environment in predicting community composition and diversity but may potentially interact with environmental factors to alter community development. We present results from a long-term community assembly experiment established in eastern Kansas and explore effects of colonization history and soil fertility on species coexistence and on the development of spatial patterns in small-scale plant species richness. In 2007, 270, 2 x 2 m experimental plots were established on abandoned agricultural land and seeded to a wide variety of different initial community states in terms of initial species pool composition, diversity and species relative abundances. Seed addition treatments were crossed with nitrogen (N) fertilization in a factorial design.

Results/Conclusions

As measured in the sixth year of the experiment (2012), species richness was significantly higher in plots originally seeded with the most species, consistent with species pool limitation. However, species richness was also strongly constrained by local competitive assortment as illustrated by substantial overall declines in species richness with time and by significantly greater rates of species loss in plots seeded with the most species, consistent with community saturation. Fertilization with N increased plant biomass, reduced light availability and reduced species richness. A marginal interaction between fertilization and species pool treatment indicated that declines in richness in response to fertilization were contingent on initial species pool diversity. Independent of the effects of N addition and species pool diversity, richness varied among plots of contrasting species composition. These preliminary results suggest that soil fertility and aspects of colonization history that determine initial community states of species composition and diversity can act independently and interactively to govern local species coexistence and species richness.